Pronounced differences between individuals in their reaction to psychostimulant drugs (amphetamine, methamphetamine, cocaine) upon first exposure suggest that certain psychophysiological condition distinguish a person at risk for stimulant abuse. Critically important for the continued desire, need, and craving for psychostimulants is the fact that these conditions may be modified by exposure to the drug itself. Both clinical and pre-clinical evidence show that chronic intake of psychostimulants can produce long-term changes in the brain reward mechanisms activated by drugs of abuse as well as by more natural forms of reward, such as food and sex. Animal studies have shown that repeated exposure to psychostimulants induces prolonged changes in motor-and reward-related dopamine systems. These changes are revealed by the increased by the increase behavioral responsiveness of animals to challenges tests with stimulants and, therefore, are collectively referred to as 'behavioral sensitization.' The finding that sensitization increase the propensity for animals to self-administer stimulants raise the possibility that changes in reward processes induced by sensitization create the psychophysiological condition of the individual most vulnerable for stimulant abuse. Work proposed here will evaluate changes in reactions by rats to fundamental 'natural' rewarding or aversive stimuli after sensitization has been induced by chronic amphetamine administration. The affective perception (pleasure/aversion) of these stimuli will be estimated in two ways: 1) responses to drinking solutions that are appetitive (sucrose) or aversive (quinine), and 2) exploration of conspecific odors that are innately appetitive (estrous, sexually receptive female) or aversive (stressed male). The incentive motivational effects (approach/avoidance) of rewarding and aversive stimuli will be estimated by a place conditioning paradigm using three types of conditioning stimuli: 1) drinking highly palatable sucrose solutions, 2) administration of a non- dopamine stimulant drug (caffeine), and 3) illness induced by lithium chloride. Reactions to these fundamental stimuli after sensitization allow direct assessment of changes in motivational processes that may exacerbate the continued need for psychostimulants.